Abstract
The search for indisputable traces of life in Archean cherts is of prime importance. However, their great age and metamorphic history pose constraints on the study of molecular biomarkers. We propose a quantitative criterion to document the thermal maturity of organic matter in rocks in general, and Archean rocks in particular. This is definitively required to select the best candidates for seeking non-altered sample remnants of life. Analysis of chemical (Raman spectroscopy, 13C NMR, elemental analysis) and structural (HRTEM) features of Archean and non-Archean carbonaceous matter (CM) that was submitted to metamorphic grades lower than, or equal to, that of greenschist facies showed that these features had all undergone carbonization but not graphitization. Raman-derived quantitative parameters from the present study and from literature spectra, namely, R1 ratio and FWHM-D1, were used to draw a carbonization continuum diagram showing two carbonization stages. While non-Archean samples can be seen to dominate the first stage, the second stage mostly consists of the Archean samples. In this diagram, some Archean samples fall at the boundary with non-Archean samples, which thus demonstrates a low degree of carbonization when compared to most Archean CM. As a result, these samples constitute candidates that may contain preserved molecular signatures of Archean CM. Therefore, with regard to the search for the oldest molecular traces of life on Earth, we propose the use of this carbonization continuum diagram to select the Archean CM samples. Key Words: Archean—Early life—Kerogen—Raman spectroscopy—Carbonization. Astrobiology 16, 407–417.
Highlights
The biological origin of some Archean carbonaceous matter (CM) is still debated (Schopf and Packer, 1987; Brasier et al, 2002; Lindsay et al, 2005; Marshall et al, 2012)
Analysis of chemical (Raman spectroscopy, 13C nuclear magnetic resonance (NMR), elemental analysis) and structural (HRTEM) features of Archean and non-Archean carbonaceous matter (CM) that was submitted to metamorphic grades lower than, or equal to, that of greenschist facies showed that these features had all undergone carbonization but not graphitization
Some Archean samples fall at the boundary with non-Archean samples, which demonstrates a low degree of carbonization when compared to most Archean CM
Summary
The biological origin of some Archean carbonaceous matter (CM) is still debated (Schopf and Packer, 1987; Brasier et al, 2002; Lindsay et al, 2005; Marshall et al, 2012). As stressed by French et al (2015), ‘‘future exploration for Archean biomarkers should screen for rocks with milder thermal histories.’’ In this respect, the thermal alteration of CM is known to be driven by two reactions, namely, carbonization and graphitization (Oberlin, 1984, 1989). Raman spectroscopy has become a favored technique with which to investigate CM evolution by way of carbonization and graphitization processes (Lahfid et al, 2010; Rouzaud et al, 2012, 2015; Charon et al, 2014). Utilizing data derived from Raman spectra in the literature, we propose a framework with which to select the most favorable samples in the search for molecular traces of life
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